This invention relates to lightweight aggregate for use with cement in connection with ultimate casting of concrete. Lightweight aggregates are old, generally, and used in making a concrete having a lower than normal density. A serious objection to prior aggregates for lightweight concrete resides not only in the lack of uniformity of characteristics of the concrete, due principally to non-uniformity of pieces of aggregate, but in addition thereto, a sub-normal compression strength in the finished concrete.
Lightweight aggregate has been prepared from cinders, pumice, Perlite, slag, shale, clay and other similar materials which are susceptible to expansion at high temperature. For the expanded product to be commercially acceptable for use in concrete, a compressive strength of the concrete from about 3,000 psi to 5,000 psi must be attained. Because of such strength requirements, the basic material used in making such aggregate has generally been limited to shale, slate or clay.
The procedures used in the production of lightweight aggregates have been generically referred to as "bloating" and materials or agents used have been generally designated as bloating agents.
Aggregate is presently produced in two general forms or manners:
1. Crushed and sized pieces of base materials (rock, shale, clay, etc.), heat treated at about 2,000.degree. F. for an appropriate period of time. PA1 2. Finely divided base materials, sometimes mixed with other ingredients, and provided with water to make a paste, which is extruded to make pellets, then heat treated as above.
In connection with form (1), the material is maintained in a dry condition until mixed with cement and water for use in making concrete. In both forms, the heat treatment suffices to drive out material as gases and, depending upon the structure of the base material, serves to reduce the density of such material. This bloating effect on the base material may vary somewhat depending upon the locality from where the base material is obtained. The characteristics of the aggregate resulting from the procedures in connection with form (1) classification, consequently may make it difficult to produce a uniform product with regard to density and, in particular, do not provide as much of a bloating action or even a uniformity of bloating action as would be required for maintaining the final characteristics of the aggregate to a reasonably satisfactory level of uniformity.
With regard to the form of aggregate set forth in (2), present practice involves the extrusion of the pasty mass through a die to create sized pellets, and heat treating such pellets. Such pellets should be porous and strong in compression but frequently lack desired porosity and are characterized by a tendency to be friable. The pellets sometimes agglomerate into large masses due to contiguous pellets caking during the heat treatment. The fine particles used in connection with form (2) are frequently obtained originally from the fines created in connection with crushing and sizing pieces of base material in connection with form (1).